In general, a bypass-type thermal flow meter includes a hot wire, i.e., a heater and a temperature sensor that are arranged in a bypass passage provided to an air intake passage of an internal combustion engine. Electricity is supplied to the hot wire, and is controlled such that a temperature difference between temperature of the hot wire and temperature detected using the temperature sensor (intake air temperature) is maintained to be constant. Thereby, the bypass-type thermal flow meter detects the amount of intake air flow in accordance with electricity supplied to the hot wire, i.e., an amount of heat radiated from the hot wire.
The thermal flow meter includes a bobbin, around which the hot wire is wound, and a support, which supports the bobbin. Heat is radiated from the hot wire to intake air, and the heat radiation delays through the bobbin, the support, and the like. Accordingly, a detection signal of the amount of intake air, which is output from the thermal flowmeter, delays in response due to delay (radiation delay) in heat radiation.
According to U.S. Pat. No. 6,644,104 (JP-A-2002-97994), the amount of intake air flow of the engine is calculated in accordance with an opening degree of a throttle valve or the like. The amount of intake air flow is compensated in accordance with the radiation delay arising delay in response (response delay) of the thermal flow meter. Thereby, the flow amount of intake air is compensated using the response delay of the thermal flow meter.
In the above bypass-type thermal flowmeter, variation in the flow amount of air, which flows through the bypass passage, delays with respect to variation in the flow amount of air, which flows through the main passage. Accordingly, the response delay arises in the air flowmeter due to the delay in the flow amount (bypass flow amount) of air flowing through the bypass passage. However, only the response delay of the air flowmeter due to the radiation delay is considered in U.S. Pat. No. 6,644,104, and the response delay of the air flowmeter due to the delay (bypass delay) in the bypass flow amount is not considered in U.S. Pat. No. 6,644,104. For example, when intake air flow is in a transient condition, and when the flow amount of intake air is small, influence of the response delay of the air flowmeter due to the bypass delay becomes relatively large with respect to the flow amount of intake air. Accordingly, in this situation, the response delay of the air flowmeter cannot be accurately compensated. As a result, accuracy in detection of the amount of intake air decreases.